Sliding parts in engines and the like are conventionally made mainly of iron-based materials for reliability. On the other hand, aluminum-based materials are increasingly used for the purpose of weight saving of parts, which contributes to reduction of CO2 emission.
Lubricants are conventionally used in internal-combustion engines and automatic transmissions for smoothing their functions. In view of the fact that heavy energy loss is suffered in friction parts, where lubricants are involved, lubricants containing a combination of various additives, including friction modifiers (FM), are used for the purpose of reducing friction losses and fuel consumption. Such reduction of friction by means of various additives has hitherto been discussed focusing on the effects on iron-based materials.
The additive compositions most effective in reducing friction losses by lubrication contain both a sulfur compound and a molybdenum-based compound (see Patent Publication 1).
However, such additive compositions have been revealed to lack sufficient effect on aluminum-based materials, which are requiring different additives and compounding techniques. It has recently been found that compounds having a hydroxyl or carboxylic group can efficiently lubricate aluminum-based materials (see Non-patent Publication 1).